154 ENTOMOLOGY 
unite to form the main lateral trunks, from which arise the 
countless branches of the tracheal system. 
Mesoderm.—I*rom the inner layer which was derived 
from the germ band by gastrulation (Figs. 189-191) are 
formed the important germ layers known as mesoderm and en- 
Fic. 199. 
Diagrammatic transverse sections to illustrate formation of dorsal wall in a beetle, 
Leptinotarsa. a, amnion (breaking up in C); g, germ band; s, serosa. After 
WHEELER, from the Journal of Morphology. 
toderm. Most of the layer becomes mesoderm, and this splits 
on either side into chambers, or cwlom sacs (Fig. 200, ¢),a pair 
to each segment. In Orthoptera these ccelom sacs are large 
and extend into the embryonic appendages, but in Coleoptera, 
Lepidoptera and Hymenoptera they are small. These sacs 
Tansverse section of germ layers of Clytra. c, celom sac; n, 
neuroblasts (primitive nervous cells).—After LECAILLON. 
may share in the formation of the definite body-cavity, though 
the last arises independently, from spaces that form between 
the yolk and the mesodermal tissues. From the ccelom sacs 
develop the muscles, fat-body, dorsal vessel, blood corpuscles, 
ovaries and testes; the external sexual organs, however, as 
well as the vagina and ejaculatory duct, are ectodermal in 
origin. 
